1. Field of the Invention
This invention relates to an anode for use in the production of electroplated metal and more particularly an improved anode for use in the continuous electroplating of elongated flat sheet and strip metal such as flat rolled steel, hereinafter sometimes referred to generally as strip. The anode of the invention is especially useful in the production of galvanized strip steel having a zinc coating on one side only.
2. Description of the Prior Art
The electroplating of strip metal is a well-known and widely used process, particularly for the plating of a corrosion-resistant coating on a base metal strip such as rolled steel. Such product is produced by passing a running length of the base strip through a bath of an electrolyte solution with the flat surface or surfaces which are to be coated passing in close parallel relation to a generally flat anode submersed in the solution. The anodes are conventionally of a generally rectangular shape and preferably have a width approximately equal to the width of the strip being plated. However, apparatus used in such plating operations is frequently employed to produce strip of different widths depending upon the intended use of the coated strip. Thus, an electroplating line designed to plate strip having a maximum width of, for example, 72 inches may be employed to process strip of substantially less than this maximum width.
The use of an anode having a width substantially greater than the width of the strip being electroplated is undesirable for several reasons. For example, the electrical energy necessary to maintain the required current density for effective electroplating will vary with the size of the anode so that energy is wasted when an anode is used which is wider than necessary. Further, the tendency to plate a heavier coating at the edges of the strip is aggrevated by an excessively wide anode, resulting in an undesirable uneveness in plating across the width of the strip.
Another problem encountered in plating strip is the greater propensity for more narrow strips to wander off the center of the desired pass line as it moves through the apparatus. This poor tracking can again result in an uneven coating across the width of the strip, particularly if it wanders to the extent that one edge projects outwardly beyond the edge of the anode.
A further problem is encountered when employing a conventional rectangular anode which is substantially wider than the base strip used in the production of one-side galvanized strip by the process disclosed in U.S. Pat. Nos. 3,988,216 and 3,989,604, which patents are assigned to the assignee of the present invention. These patents disclose a process in which a strip, previously galvanized on both sides, is passed through a bath of electrolyte solution between an anode and a separate cathode so as to remove the zinc coating from one side of the strip while simultaneously electrodepositing a substantially equivalent amount of zinc on the opposite side. In producing one-side galvanized strip according to the method of those patents, there is a tendency for zinc to re-plate around the edges of the strip onto the side thereof from which the zinc has been removed when the high current densities required for high speed production is used in an apparatus having an anode substantially wider than the strip being coated.
It would, of course, be possible to change the anode in a strip-plating apparatus each time that a strip of a different width is processed. However, this is not a practical solution in a high-speed production line because the time required to affect the change-over would be prohibitive. Further, the anodes are normally connected to a source of electrical current by bus-bars which are designed to supply current across the width of the anode so that it would normally be necessary to change or modify the bus-bars when changing from one size anode to another. Also, the anodes are heavy and difficult to handle, and are easily damaged.